The predictions of Albert Einstein they still amaze the scientific community more than a century after he formulated them, both those already confirmed and those we continue to explore.
Albert Einstein is at the top of the list of the most famous and iconic scientists in history. His theories of Special Relativity in 1905 and General Relativity in 1915 literally revolutionized physics.
It went beyond Newton’s theory of gravity, which had been around since 1687. Einstein also introduced his famous thought experiments, which also tested early developments in quantum mechanics. His contributions in this field deserved the Nobel Prize in Physics, that they gave him in 1921 for him photoelectric effect.
Many people believe that the Nobel Prize for General Relativity, which they did not give him, is a great outstanding debt. In this new theory, gravity is understood as a deformation or curvature of space-time, caused by the distribution of masses and energies.
The more mass that is packed into less volume, the more space-time around it is warped or curved. Any other particles or objects that pass near these objects feel this curvature, which causes their trajectory to change.
Confirmed prediction: the day the curvature of space-time was observed
Some of the predictions or consequences of General Relativity were put to the test in a short time. In 1919, only 4 years after the publication of the theory, a total solar eclipse took place. It was the ideal event to test the curvature of space-time.
There were several scientific expeditions that traveled to Brazil and the West African coast to take the best photos and data of that eclipse and, above all, of the stars that surrounded the Sun.
The most massive and compact object that we have in our vicinity is the Sun. What was wanted check was whether the light from distant stars was affected by the curvature of space-time generated by the Sun passing close to it.
If so, its path would deviate slightly from a straight line, causing the star’s apparent position in the sky to change slightly. Confirmation of this effect, consistent with measurements of the 1919 eclipse, made Einstein world famous.
Einstein’s doubts: the vibrations of space-time
To experimentally demonstrate other predictions of General Relativity we have needed to wait much longer. In 1916 Einstein began to analyze in great detail his equations, and in particular a series of terms that, after a small simplification, look remarkably like a wave equation: the same structure that appears in many physical systems where we have a disturbance that is propagates by transporting energy.
In this case, the equations say that what vibrates is space-time itself, and we call these disturbances gravitational waves.
Could they be seen? would there be any way “hear” the vibrations of space-time?
During his life, Einstein doubted the real existence of this phenomenon (perhaps it was a mathematical artifact but without physical realization?). Einstein was not the first or the only physicist to doubt the mathematical consequences of his theory. He had his ups and downs with colleagues and prestigious scientific journals that have given rise to very interesting stories.
Be that as it may, and with the contribution of prominent personalities, it was finally understood that the gravitational waves were an actual prediction of the theory.
Their properties were analyzed and it only remained to be seen if the technological race to experimentally verify their existence bore fruit.
Confirmed prediction: gravitational waves were “heard” at last
The amplitude of these waves is so so so (you can put all the “tans” you want) extremely weak that Einstein himself did not have much confidence that its detection would one day be possible.
Each of the tests to which General Relativity was subjected was not capable of finding discrepancies, but not detecting gravitational waves or detecting them with properties different from those theorized would be a demonstration that this theory did not faithfully reproduce reality: the glove was lying.
The success of technological development took decades, and the usual failed attempts that are not always mentioned in science, such as the pioneering experiments of the physicist Joseph Weber with resonant bars in the 1960s.
The instruments that have been able to finally overcome this challenge are the kilometric arm laser interferometers.
The first detection of gravitational waves took place in 2015was carried out by the American observatories LIGO and it was a literally historic event.
The detected gravitational waves were also associated with another of the consequences of General Relativity: they came from the merger of two black holes of about 36 and 29 times the mass of the Sun, and passed through the detectors after traveling about 1,300 million light-years.
The European Virgo observatory joined the data collection in the summer of 2017, with a triple detection of a neutron star merger that included gravitational waves in multi-messenger astronomy. The KAGRA observatory will join the global network in the next observing period, scheduled for December this year.
We already have a total of 90 confirmed eventsall of them have as astrophysical scenario the merger of two compact objects: pairs of black holes, pairs of neutron stars or mixed pairs of a black hole and a neutron star.
The door of research is open to compact objects of a different nature, and the gravitational waves they generate can give us clues about their structure and properties. We are impatient to see the new surprises that are to come.
The cosmological constant: Einstein’s biggest “blunder”?
In the chapter on Einstein’s predictions we cannot forget the famous cosmological constant, which also generated contradictions. This constant, its properties and whether it is capable of faithfully modeling the evolution and expansion of the universe in the light of future data is the page of the book that is being written right now.
Einstein introduced this constant in his equations to force (by personal beliefs) a model of static universea kind of “repulsive energy” without which the universe would end up collapsing due to the effect of gravity itself.
However, after the observations in 1931 of the physicist Edwin Hubble on the expansion of the universe, Einstein considered his proposal as “the biggest blunder” of his scientific work. Was it really?
The interest in the cosmological constant introduced by Einstein resurfaced with quantum field theoriessince they predict a vacuum energy that can behave, for all purposes, like the cosmological constant that he predicted.
So it seems that Einstein, again, got it right again.